This invention relates to a discharge lamp adapted for operation by an alternating current. More particularly, the invention relates to a control circuit which optimizes the start-up and normal operation of the discharge lamp with a minimum number of circuit components.
A prior art discharge lamp circuit using a voltage multiplier control circuit is disclosed in U.S. Pat. No. 4,010,398. A disadvantage of this known lamp control circuit is that it uses a capacitor having a fixed value so that at the beginning of the starting procedure -- when the lamp temperature is still substantially equal to the ambient temperature -- the lamp control circuit has the same properties as when the lamp is in operation. This means that the lamp control circuit does not have the optimum capacitance value for all circumstances.
Another prior art discharge lamp control circuit is described in U.S. Pat. No. 3,996,495 wherein a voltage and/or current dependent capacitor is connected in circuit so as to improve the crest factor of the lamp current waveform and hence the operating efficiency of the system. U.S. Pat. No. 4,029,989 describes a discharge lamp having a separate starting electrode to which a voltage doubler circuit is connected which includes a temperature-dependent capacitor. Neither of these systems automatically provides optimum operation of the discharge lamp in both the start-up period and during normal lamp operation subsequent to the ignition thereof.
It is an object of the invention to provide a control circuit for a discharge lamp wherein the capacitor has a suitable capacitance value in the various conditions of the lamp.
A further object of the invention is to provide a discharge lamp control circuit operable from a source of AC current and including a temperature-dependent capacitor that automatically varies its capacitance value as a function of the condition of the lamp so as to provide optimum values of capacitance for all conditions of the lamp.